A water absorbent resin obtained by polymerizing an unsaturated monomer containing a carboxyl group is widely used for various applications such as application as sanitary materials, e.g., disposable diapers for babies, sanitary napkins, and incontinent pads, application as a cable waterproofing agent, application in agriculture/horticulture, application in civil engineering and construction, and food related application. The water absorbent resin is typically obtained as a result of processes including a step of drying a cross-linked polymer hydrogel crushed to a size easy to dry, a pulverization step, a classification step, a surface cross-linking step, and the like. The cross-linked polymer hydrogel is obtained by polymerizing an unsaturated monomer containing a carboxyl group in the presence of an internal cross-linking agent.
In the production of the water absorbent resin, a method used as a method of crushing the cross-linked polymer hydrogel to a size with which the cross-linked polymer hydrogel is easily dried is one or a combination of a method of concurrently polymerizing and crushing in a reaction container including crushing means and a method of crushing the cross-linked polymer hydrogel by a crusher or the like after polymerization without crushing.
In particular, the method of crushing the cross-linked polymer hydrogel by a crusher or the like after polymerization is superior in that the method makes it possible to crush the cross-linked polymer hydrogel to a size with which the cross-linked polymer hydrogel obtained can be efficiently dried reliably and uniformly.
For example, FIG. 1a shows one example of a method in which the cross-linked polymer hydrogel is crushed by a crusher after a monomer is polymerized without crushing. As illustrated in FIG. 1a, a band shaped cross-linked polymer hydrogel 3 obtained by polymerization in a polymerization apparatus 1 is crushed by a crusher 2, thereby providing a crushed cross-linked polymer hydrogel that is a jelly-like crushed articles of the cross-linked polymer hydrogel.
For production of a water absorbent resin obtained by polymerizing an unsaturated monomer containing a carboxyl group, some reports deal with a method of using, as an internal cross-linking agent, a combination, in a predetermined proportion, of (i) an internal cross-linking agent having at least two radical polymerizable unsaturated groups in its molecule and (ii) an internal cross-linking agent having in its molecule at least two functional groups that can react with a carboxylic group (Refer to, for example, Patent Documents 1 through 6).
For example, Patent Document 1 discloses a technique in which (i) a compound having at least two unsaturated groups in its molecule and (ii) a compound having at least two epoxy groups in its single molecule as functional groups that can react with a carboxylic group are used in a specific proportion in polymerization, for the purpose of producing a water absorbent resin that has excellent absorbency under pressure and low extractable polymer content.
Moreover, Patent Document 2 discloses a technique in which a cross-linking agent containing a polyvinyl first cross-linking agent and a hydroxyl-containing second cross-linking agent are used in polymerization for the purpose of producing a water absorbent resin having a high gel bed permeability (i.e. liquid permeability) and a low absorption capacity.
Further, Patent Document 3 discloses a technique in which a compound containing at least two unsaturated groups in its single molecule and alkylene carbonate are used in polymerization for the purpose of providing a water absorbent resin generating sound in water-absorption.
In addition, Patent Documents 4 through 6 disclose a technique in which an acrylic ammonium salt, an internal cross-linking agent having at least two unsaturated groups in its single molecule, and a condensation-type cross-linking agent such as glycerin are used for the purpose of improving absorbing performance.    [Patent Document 1] Japanese Unexamined Patent Publication No. 188602/1996 (Tokukaihei 8-188602) (published on Jul. 23, 1996)    [Patent Document 2] Japanese Unexamined Patent Publication No. 518150/2003 (Tokukai 2003-518150) (published on Jun. 3, 2003)    [Patent Document 3] Japanese Unexamined Patent Publication No. 200168/1994 (Tokukaihei 6-200168) (published on Jul. 19, 1994)    [Patent Document 4] Japanese Unexamined Patent Publication No. 200630/2005 (Tokukai 2005-200630) (published on Jul. 28, 2005)    [Patent Document 5] Japanese Unexamined Patent Publication No. 199805/2006 (Tokukai 2006-199805) (published on Aug. 3, 2006)    [Patent Document 6] Japanese Unexamined Patent Publication No. 225455/2006 (Tokukai 2006-225455) (published on Aug. 31, 2006)